Shutters that control both the aperture and exposure time have been used to regulate the amount of light that reaches the imaging capture area of a filmstrip contained within a camera or other imaging devices like scanners, printers etc. Open loop systems have been utilized as taught by the prior art to control the operation of the shutter blades. In an open loop control system the input to the shutter is a complex waveform that is tailored to a particular shutter, and no information about the location of the shutter blades is provided to the electronics that controls the shutter. However, the characteristics of a shutter change with time. This is due to changes in friction of the shutter's components which is caused by contamination and wear or temperature changes of the shutter.
A closed loop system is disclosed in Japanese Patent Application No. 61-158586 entitled "Shutter Controller For Camera". This application utilizes a variable size slot 31, that is positioned on a shutter blade actuator ring to attenuate light passage from a LED to a photosensor. The slot, LED and photosensor are aligned in a manner that the slot changes position and size as the actuator ring moves and changes the shutter blades position on the actuator ring.